The determination of trace components of gases and especially of the SO.sub.2 content in exhaust gases has become one of the most important questions of measuring technique of the present time.
All of the presently known measuring apparatus which operate on various principles (for example infrared, thermal conductivity, UV etc.,) are either too expensive or can only be utilized within rather narrow ranges.
It is possible to show by comparative experiments that a measuring device based upon electrochemical principles can not only be constructed very economically but is also utilizable over rather wide concentration ranges.
Furthermore by appropriate selection of the working potential it is possible to contemporaneously measure a variety of oxidisable or reducible materials, which is a further advantage.
An electrochemical measuring cell for SO.sub.2 has been constructed heretofore (Chemie.-Ing Technik 49, 398 (1977)). This cell was conceived as a modification of a conventional fuel cell, that is to say, it operates with porous electrodes. This fact gives rise to a non negligible complication. Since the measuring, as well as the counter electrodes have approximately similar polarization qualities, such a cell must be operated with a potentiostat and a 3 electrode system (i.e. measuring-counter-and reference electrode). This is a further complication which must of course lead to an increase in cost of the device. The rapidity of the signal change, with respect to a concentration of sulfur dioxide which alters over a given time period, is not known. It must be presumed, however, that there would be a fairly substantial time delay since the diffusion of the gas must clearly be somewhat delayed by the inert Teflon underlayer.
A further method of measuring sulfur dioxide concentration by electrochemical means is disclosed by J. V. A. Novak (Collection Czech Chem. Commun. 25, 3099 1960).
This method is based on the concept of the change of the diffusion restricted polarographic step of sulfide dioxide dissolved in an electrolyte. A carbon rod is utilized as the indication electrode. As the unpolarizable counter electrode there is utilized metallic copper in a solution of cupric ions (copper sulfate). This analyzer is very steady, simple in concept substantially disturbance free in practice, and also fairly inexpensive.
There are, however, problems inherent in the construction of a such a device. The gas is fed sidewards to a measuring electrode having an angled surface cut therein. This construction, because of unsatisfactory electrolyte flow, leads to an unfortunate sluggishness in the system. When the concentration of the sulfur dioxide alters suddenly, the corresponding signal change is only noted with a considerable time delay (See FIG. 2. Curve A shows actual SO.sub.2 concentration change and Curve B the signal from a device constructed in accordance with the Novak disclosure.)
The surface of the electrode directly available to the gas stream is so small that the measured current which is the indicator of the sulfur dioxide concentration is also relatively small. Thus, clearly the obtainable measuring sensitivity is also not great (See FIG. 3, curve A). If one increases the electrode surface (i.e. by using a thicker rod) in order to avoid this disadvantage, the measuring signal is increased, however, the electrode surface directly available to the gas stream still remains relatively small and the already unsatisfactory signal delay is thus made worse.